I stumbled on a good mechanical filter article that goes into upgrading the SX-42 and SP-600. I found this pretty interesting.
This is how it starts:
A complaint many shortwave receiver owners have is the inadequate skirt selectivity in their equipment. This is not just a problem in old or inexpensive units. The skirt selectivity of even some modem, top-priced receivers is less than what it could be for dealing with current crowded band conditions. Mechanical filters are often expensive, and they may need accompanying resonating capacitors. Sometimes a good drop-in substitute filter can be found; otherwise installation may be difficult. Even with a new filter successfully in place, an insertion loss can result because of impedance mismatch. Such a loss, averaging 6 dB, is not important for strong signals, but for weak ones it makes all the difference in readability.
http://www.kiwa.com/goodwin.htm
The -6/-6OdB band widths for the SX-42 are about 4/21 kHz and for the SP-600J about 3/18 kHz - good in their respective times, but most inadequate for today's conditions.
SP-600:
'3' means 3.0 Khz at -6db, and 18 Khz at -60db
'8' means 8.0 Khz at -6 db, and 26 Khz at -60db
'13' means 13.0 Khz at -6db, and 35 Khz at -60db
Jim
JKO